8185

• UCREFRP Catalog Number: 8185
• Author: Valdez, R. A. and B. R. Cowdell.
• Title: Annual Summary Report-1992\
• USFW Year: 1993.
• Copyright Material: NO

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1 ~FALL SAMPLING .SPRING SAMPLING I

300

w
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<...:> 250
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::ii: 200
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E 150
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~ 100
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<, ~<::s

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Five-Mile River Sections

Figure 4. Cumulative geometric mean CPE for all five-mile river sections sampled in Green River Reach
3 during fall and spring sampling (1987 through 1992 combined).

4.1.3 Ho 3: A2e-O Colorado sQuawtish are lost to ovenvinter mortality from starvation. disease. parasites.
etc.

Hypothesis 3 suggests that decreases in CPE of age-O Colorado squawfish between fall and spring represent
mortality of fish during the winter period. There is considerable evidence that winter riverine conditions
significantly influence cohort strength of Colorado squawfish. This impact may be most significant in the first
year of life, when age-O fish are exposed to winter conditions at less than 4 months of age, and often less than
50 mm in length. Kaeding and Osmundson (1988) stated that the unusually small size of age-O fish going into
the winter might be an important factor affecting recruitment to adult stock. Thompson (1989) determined
that age-O Colorado squawfish entering the winter period at lengths greater than 35 mm had a higher fat
content and presumably have a greater chance of overwinter survival.

Low survival of age~O Colorado squawfish during the 6-month winter period is attributed to: (1) competition
and predation by large numbers of non-native species, (2) diseases and parasites, and (3) wintertime icing of
backwaters and exclusion of low-velocity habitats. Factors 1 and 2 were not addressed in this report, but are
recognized as potentially significant to cohort strength. Harsh and changing river conditions faced by age-O
squawfish probably account for a substantial portion of the mortality observed during the winter months.

Ice formation and cold temperatures may force age-O Colorado squawfish out of backwaters into less suitable
habitat in the mainchannel. Movements and habitat use of squawfish during the winter period are unknown.
However it was hypothesized that if backwaters provided sheltering habitat during the winter, forced
abandonment would increase mortality of age-O squawfish. To test this hypothesis all data for Reach 3 were
pooled and geometric mean CPE compared between fall and spring for varying backwater depths (Table 8).

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